All domesticated plant and animal species are genetically modified. The only way you can refrain from eating genetically modified food is to hunt wild game and gather wild roots and berries and eat these non-genetically-modified foods.
So, it is much more reasonable to label wild animal flesh and wild plant species: "Not Genetially Modified." But good luck trying to find anything wild in your local grocery store.
Do you really want to label every single item of food in a grocerty store as geneetically modified? Or do you think that using molecular biology to genetically modify an organism is fundamentally different than using selective breeding (what humans have been doing for thousands of years)? If you do, you would fail an introductory biology course.
Since the domestic dog is a genetically modified wolf aare you going to hang a label on your pet dog?